I think one of the creepiest elements about the Gentlemen is why they do things. Because the answer to that is we have no idea.
They roll up in a town, seemingly randomly, steal everyone's voices, select seven victims, and given they specifically choose them, this isn't random, but we don't know why it isn't, cut out seven hearts, which they need for some unknown reason, then leave, unless someone stops them. Why do they do this? No-one knows.
I hadn't thought of what the Gentlemens motivation was before, I think I had assumed it was to feed their spell or some God demon.
What do you think their motivation was to come to Sunnydale and other towns to steal hearts?